Since a back light provided to a projection type image display device such as a projection type liquid crystal display or a liquid crystal projector is required to project images uniformly at a sufficient luminance, efficiency, and color rendition to a rectangular screen, a high pressure discharge lamp such as a high pressure mercury lamp or metal halide lamp is used as a light source.
Generally, in the lamp of this type, mercury as a light emitting substance, a halogen compound such as a bromide that performs the halogen cycle effect of returning tungsten evaporated from the electrodes and deposited to the inner surface of the discharge vessel to the electrodes thereby suppressing blacking of the arc tube, and a rare gas such as argon, krypton or xenon as an auxiliary gas for starting lighting are sealed in a discharge vessel of the arc tube in which a pair of electrodes comprising tungsten are opposed. In the short arc type high pressure discharge lamp with the inter-electrode distance being shortened in order to provide a high luminance light source approximate to a spot light source, since the temperature at the top end of the electrode increases excessively, it results in a problem of remarkable melting and evaporation of tungsten to deform or abrade the top end of the electrode, as well as early blackening of the arc tube by deposition of evaporated tungsten to the inner surface of the discharge vessel to shorten the lamp life.
Further, when the inter-electrode distance is shortened, it results in an arc jump phenomenon that an arc spot (region to which an electron current is emitted upon cathode operation of the electrode) formed in the initial lighting period of the lamp near the center at the top end of the electrode moves cyclically (on every lighting period) to a position different from the initial lighting period along with lapse of the lighting time and the screen illuminance of the protection type image display device is lowered as much as by about 30% during lighting only for about 100 hours, or unpleasant flicker is caused on the screen due to fluctuation of the luminance.
That is, in the high pressure discharge lamp used as a light source for the backlight of the projection type image display device, while optical axes are previously adjusted so as to align the tubular axis of the arc tube with the central axis of a concaved reflection mirror for reflecting the light emitted from the arc tube, when the arc jump phenomenon should occur, since the arc spot displaces from the optical axis and moves at random on every lighting period, this results in a problem of unpleasant flicker on the screen due to fluctuation of the luminance and lowering the screen illuminance.
In view of the problem described above, JP-A Nos. 2001-312997, 2001-325918, and 2002-83538 disclose techniques of diametrically enlarging the top end of the electrode to increase the heat capacity in order to suppress melting and evaporation of electrode-forming. tungsten, and forming the top end of the electrode into a substantially semispherical shape to form a region of generating the arc spot at a spherical convex surface in order to suppress the occurrence of the arc jump phenomenon.
Further, JP-A No. 2001-312997 discloses a technique of supplying AC current for a predetermined time between a pair of electrodes, each top end of which is formed into a substantially semispherical shape to generate arc discharge and previously forming a protrusion as an arc spot at each top end of both electrodes by the discharge thereby increasing the heat capacity at the top end of the electrodes and preventing the occurrence of the arc jump phenomenon.
However, according to the experiment and study on the function and the effect of this technique, it has been found that unless various conditions such as the lighting waveform or the lighting frequency of the lamp, the bromine concentration of the bromide sealed in the discharge vessel of the arc tube, the inter-electrode distance, the level of the lamp current, and the weight of tungsten at the top end of the electrodes each formed into the substantially semispherical shape are controlled properly, the protrusion as the arc spot formed at the top end of the electrode is eliminated only in several minutes from the start for the lighting of the lamp to cause early arc jump phenomenon, or the protrusion grows abnormally to shorten the inter-electrode distance thereby bringing about a disadvantage of lowering the lamp voltage in an early stage and lowering the illuminance and, further, that deformation or abrasion at the top end of the electrode and early blackening of the arc tube are caused.
In view of the above, the present invention intends to properly control the conditions described above in the high pressure discharge lamp, thereby preventing the deformation or abrasion at the top end of the electrode and early blackening of the arc tube and suppressing elimination and abnormal growth of the protrusion as the arc spot formed to the top end of the electrode to reliably prevent occurrence of the arc jump phenomenon and prevent flicker on the screen of the projection type image display device using a high pressure discharge lamp as a light source and prevent lowering of the screen illuminance.